1. Field of the Invention
The present invention relates to a process and apparatus for recovering hydrocarbons from air-hydrocarbon vapor mixtures, and more particularly, but not by way of limitation, to an improved process and apparatus for recovering vaporized gasoline light ends and the like from a mixture thereof with air expelled from tank cars, trucks and the like.
2. Description of the Prior Art
In handling multi-component hydrocarbon liquids such as gasoline, kerosene and the like, air-hydrocarbon vapor mixtures are readily produced which cannot be vented directly to the atmosphere due to the resulting pollution of the environment and fire and/or explosion hazard. Consequently, a variety of processes and apparatus have been developed and used for removing hydrocarbon vapors from such air-hydrocarbon vapor mixtures whereby the remaining air can be safety vented to the atmosphere. The removed hydrocarbons are generally liquefied and recombined with hydrocarbon liquids from which they were vaporized thereby making the recovery economically advantageous.
A process for the recovery of light mixed hydrocarbon vapors from an air-hydrocarbon mixture expelled as a result of storage breathing or loading of vented hydrocarbon vessels is described in U.S. Pat. No. 4,066,423. In accordance with such process, the air-hydrocarbon vapor mixture from which hydrocarbons are to be removed and recovered is passed through a bed of solid adsorbent having an affinity for hydrocarbons. As the mixture passes through the bed, a major portion of the hydrocarbons contained in the mixture are adsorbed on the bed and a residue gas stream is produced which is comprised of substantially hydrocarbon-free air. The hydrocarbon-rich air-hydrocarbon mixture produced as a result of the regeneration of the bed is contacted with a liquid absorbent whereby hydrocarbons are removed therefrom and the residue gas stream from the absorption step is recycled to the bed through which the inlet air-hydrocarbon mixture is flowing. In accordance with the teachings of U.S. Pat. No. 4,066,423, the liquid absorbent utilized is liquid hydrocarbons condensed from the air-hydrocarbon vapor mixture produced in the evacuation regeneration step. More specifically, the hydrocarbon-rich air-hydrocarbon vapor mixture is cooled whereby portions of the hydrocarbons are condensed and such condensed hydrocarbons are circulated into contact with the remaining air-hydrocarbon vapor mixture whereby hydrocarbon vapors are absorbed by the liquids. The use of hydrocarbons which are condensed from the hydrocarbon-rich air-hydrocarbon vapor mixture to contact the remaining air-hydrocarbon vapor mixture results in the inefficient absorption of hydrocarbons from the remaining air-hydrocarbon vapor mixture. Numerous other processes and apparatus for recovering hydrocarbons from air-hydrocarbon vapor mixtures or otherwise treating said mixtures are disclosed in U.S. Pat. Nos. 3,897,193; 3,768,232; 3,867,111; 3,455,089, 3,543,484; and 3,776,283.
A problem which has not been addressed or solved in any of the prior art processes or apparatus is the overheating of the beds of solid adsorbent due to the reaction of hydrocarbons contained in the inlet mixture of air and hydrocarbons with air. It has been found that beds of solid adsorbent through which air-hydrocarbon mixtures are flowed for the purpose of adsorbing and removing the hydrocarbons from the mixture can reach conditions whereby runaway heating of the beds takes place bringing about an extremely dangerous condition, i.e., a condition whereby the air-hydrocarbon mixtures within the beds as well as the solid adsorbent itself can auto-ignite.
The runaway overheating of the beds of solid adsorbent is most prone to take place in applications wherein large adsorbent beds are used, high ambient temperatures exist, and where the air-hydrocarbon vapor mixtures being processed are mixtures expelled from the loading of vented hydrocarbon vessels such as tank cars, tank trucks, etc. From these vessels hydrocarbon concentrations in the air-hydrocarbon vapor mixture are high and may contain other hydrocarbon impurities besides those for which the processing system was designed due to previously hauled substances. In addition, the nature of this service requires the adsorption beds to be operated at very low superficial vapor velocities which inhibit heat dissipation. The combination of the heat released by the adsorption of hydrocarbons, high initial bed temperatures, exothermic hydrocarbon reactions with air, and hindrances to heat dissipation caused by large adsorption vessels and low vapor velocities has resulted in accelerated heating of the beds, i.e., as the temperature of the beds increases greater quantities of materials react causing the temperature to increase at an accelerated rate and rapidly reach an overheated and extremely dangerous condition. In addition, the overheating of the beds of solid adsorbent can severely reduce the efficiency and prevent proper operation of apparatus of which the beds are a part.
By the present invention an improved process is provided which prevents overheating of the beds of solid adsorbent and which utilizes a stream of liquid from which the hydrocarbon vapors originated to absorb and recover the removed hydrocarbons.